Reference signal interference management in heterogeneous network deployments

ABSTRACT

Methods and apparatus for enabling interference coordination in a communication network. A base station includes a plurality of antenna ports. Each antenna port is configured to transmit a reference signal, and each antenna port is associated with a respective cell. The base station determines a set of cells where transmissions of reference signals is to be performed from a reduced set of the plurality of antenna ports. The base station determines a subset of antenna ports in at least one cell of the determined set of cells to enable interference coordination in the network, and transmits the reference signal from the subset of antenna ports.

TECHNICAL FIELD

The embodiments herein relate in general to signal measurements inwireless communications networks and in particular to managing andcoordinating the interference from reference signals in heterogeneousnetwork deployments.

BACKGROUND

In a typical cellular system, also referred to as a wirelesscommunications network, wireless terminals, also known as mobilestations and/or User Equipment units (UEs) communicate via Radio AccessNetworks (RAN) to a Core Network (CN). The wireless terminals may bemobile stations or user equipments such as mobile telephones also knownas cellular telephones, and laptops with wireless capability, e.g.,mobile termination, and thus may be, for example, portable, pocket,hand-held, computer-included, or car-mounted mobile devices whichcommunicate voice and/or data with radio access network. The radioaccess network covers a geographical area which is divided into cellareas, with each cell area being served by a radio network node, such asa base station, which in some radio access networks is also calledeNodeB (eNB), NodeB, or base station. A cell is a geographical areawhere radio coverage is provided by the radio base station at a basestation site. Each cell is identified by an identity within the localradio area, which is broadcast in the cell. The base stationscommunicate over the air interface operating on radio frequencies withthe user equipments within range of the base stations. There aredifferent types of radio network nodes/base stations, such as forexample macro node/base station, pico node/base station, home eNodeB orfemto base station. Typically, the types of base stations are associatedwith different power classes, e.g. a typical maximum transmit power ofmacro base station (aka wide-area base station) is above 40 dBm, whilstlower-power base stations such as pico of femto typically have theoutput power below 30 dBm.

The interest in deploying low-power nodes, such as pico base stations,home eNodeBs (HeNB, HBS), relays, remote radio heads, etc., forenhancing macro network performance in terms of the network coverage,capacity, and service experience of individual users has been constantlyincreasing over the last few years. At the same time, it has beenrealized the need for enhanced interference management techniques toaddress the interference issues caused, for example, by a significanttransmit power variation among different cells and cell associationtechniques developed earlier for more uniform networks.

In 3^(rd) Generation Partnership Project (3GPP), heterogeneous networkdeployments have been defined as deployments where low-power nodes ofdifferent transmit powers are placed throughout a macro-cell layout,implying also non-uniform traffic distribution. Such deployments are,for example, effective for capacity extension in certain areas,so-called traffic hotspots, i.e., small geographical areas with higheruser density and/or higher traffic intensity where installation of piconodes can be considered to enhance performance. Heterogeneousdeployments may also be viewed as a way of densifying networks to adoptfor the traffic needs and the environment. However, heterogeneousdeployments bring also challenges for which the network has to beprepared for to ensure efficient network operation and superior userexperience.

In heterogeneous networks, a mixture of cells of differently sized andoverlapping coverage areas are deployed. A cell is a geographical areawhere radio coverage is provided by a base station. More than one cellcan be associated with one base station. One example of such celldeployment may be a network comprising pico cells deployed within thecoverage area of a macro cell. The pico cells and macro cell may eachcomprise a base station. A base station may be e.g. a pico base station,a macro base station, Home Base Station (HBS), radio base station,evolved node B (eNB), base station, relay, remote radio heads etc.

A base station comprises at least one antenna port, e.g. antenna port 0.Each antenna port is configured to transmit and receive signals from thebase station to e.g. one or more user equipment.

Other examples of low-power nodes in heterogeneous networks are homebase stations (HBS) and relays. As discussed below, the large differencein transmitted output power, e.g., 46 dBm in macro cells and less than30 dBm in pico cells, results in an interference situation differentfrom that seen in networks where all base stations have the same outputpower.

A Long Term Evolution (LTE) system uses Orthogonal Frequency DivisionMultiplex (OFDM) as an OFDM Access technique (OFDMA) in the downlinkfrom system nodes to user equipments (UEs) 505, and Discrete FourierTransform (DFT)-spread OFDM in the uplink from a user equipment 505 toan eNB. LTE channels are described in 3GPP Technical Specification (TS)36.211 V9.1.0, Physical Channels and Modulation is described in Release9 of LTE, among other specifications. An LTE system is used as anexample in this document. However other network standards, such as GPRS,WiMAX, UMTS etc. are also applicable.

In the time domain, LTE downlink transmissions are organized into radioframes of 10 milliseconds (ms) duration, each radio frame 101 comprisesten equally-sized subframes 103 of 1 ms duration as illustrated inFIG. 1. A subframe 103 is divided into two slots, each of 0.5 msduration. Time domain is a term used to describe the analysis ofphysical signals, with respect to time.

The resource allocation in LTE is described in terms of resource blocks,where a resource block corresponds to one slot in the time domain and 12contiguous 15 kHz subcarriers in the frequency domain. Two consecutive,i.e. in time, resource blocks represent a resource block pair andcorrespond to the time interval upon which scheduling operates.

Reference Signals

The use of multiple antennas plays an important role in modern wirelesscommunication systems, such as 3^(rd) Generation (3G) LTE systems, toachieve improved system performance, including capacity and coverage,and service provisioning. Acquisition of channel state information (CSI)at the transmitter or the receiver is important to proper implementationof multi-antenna techniques. In general, channel characteristics, suchas the impulse response, are estimated by sending and receiving one ormore predefined training sequences, which can also be called ReferenceSignals (RS). To estimate the channel characteristics of a DL forexample, a base station 503 transmits reference signals to userequipments 505, which use the received versions of the known referencesignals to estimate the DL channel, e.g. to provide an estimated channelmatrix. The user equipments can then use the estimated channel matrixfor coherent demodulation of the received DL signal, and obtainpotential beam-forming gain, spatial diversity gain, and spatialmultiplexing gain available with multiple antennas. In addition, thereference signals may be used to do channel quality measurement tosupport link adaptation.

Beam-forming is a signal processing technique used to control thedirectionality of the reception or transmission of a signal. Spatialdiversity refers to using two or more antennas to improve the qualityand reliability of a wireless link. Using multiple antennas offers areceiver several observations of the same signal. Spatial MultiplexingGain is obtained when a system is transmitting different streams of datafrom the same radio resource in separate spatial dimensions. Data ishence sent and received over multiple channels—linked to different pilotfrequencies, over multiple antennas.

Transmissions in a network using OFDM may be seen as a grid in time andfrequency. The scheduler in the base station may allocate a specificnumber of subcarriers during a specific time to one user equipment. Tosimplify the system, too small units cannot be allocated to one userequipment, and the smallest unit within OFDM is referred to as aresource element, and that is one OFDM symbol transferred on onecarrier. In the case of OFDM transmission a straightforward design of areference signal is to transmit known reference symbols in an OFDMfrequency-vs.-time grid. Cell-specific Reference Signals (CRS) andsymbols are described in Clauses 6.10 and 6.11 of 3GPP TS 36.211. Up tofour cell-specific reference signals corresponding to up to fourtransmit antennas of an eNodeB are specified. There is one referencesignal transmitted per downlink antenna port. Among the aforementionedreference signals, only CRS have to be transmitted in every downlinksubframe, and the other RS are transmitted at specific occasionsconfigured by the network.

LTE uses four types of downlink reference signals (RS):

-   -   Cell-specific reference signals, associated with non Multimedia        Broadcast/Multicast Service Single Frequency Network (MBSFN)        transmission.    -   MBSFN reference signals, associated with MBSFN transmission.    -   UE-specific reference signals.    -   Positioning reference signals.

The reference signals are referred to as RS in some of the figures.

Cell-Specific Reference Signals

CRS are transmitted in the downlink from an eNB, or base station, to auser equipment, or terminal, every subframe and over the entire systembandwidth, from antenna ports 0, 1, 2 or 3. In non-MBSFN subframes,cell-specific reference signals (CRS) are transmitted on the resourceelements shown in FIGS. 2 a-c, for the case of a normal cyclic prefix.In telecommunications, the term cyclic prefix refers to the prefixing ofa symbol with a repetition of the end. In subframes used for MBSFNtransmissions, only the first two symbols may be used for CRS. FIGS. 2a-c illustrates a resource grid of sub-carriers and available OFDMsymbols for antenna ports. Each element in the resource grid is called aresource element. Each resource element is used to transmit a referencesignal on one antenna port.

FIG. 2 a illustrates CRS transmission from one antenna port, FIG. 2 billustrates CRS transmission from two antenna ports and FIG. 2 cillustrates CRS transmission from four antenna ports. The x-axis of theFIGS. 2 a-c are time slots In FIGS. 2 a-c, the notation Rp is used todenote a resource element used for reference signal transmission onantenna port p. The hatched resource elements without any text indicateresource elements which are not used for transmission on the antennaport of interest. The hatched resource elements with text, Rp, indicatereference symbols transmitted on the antenna port of interest. Forexample, in FIG. 2 b, reference signals R1 is located in the first OFDMsymbol (1^(st) RS) and 3^(rd) to the last OFDM symbol (2^(nd) RS).

Different cells can use 6 different shifts in frequency, and 504different signals exist. The frequency shifts are cell-specific anddepend on Physical layer Cell Identity (PCI). The relation between thePCI and the CRS frequency shift is given by V_(shift)=N_(ID) ^(cell) mod6, i.e., formally up to six-reuse may be configured for CRS. Inpractice, however, the effective reuse depends on the number oftransmitting antenna ports. As may be seen from FIG. 3, CRS have areuse-six pattern for CRS transmitted from 1 antenna port andreuse-three for 2 to 4 antenna ports.

CRS measurements are used at least for control channel demodulation,mobility measurements, e.g. Reference Signal Received Power (RSRP) andReference Signal Received Quality (RSRQ), and channel estimation. Whenmeasuring RSRP and RSRQ, the user equipment 505 measures over a measuredbandwidth, which can be smaller than the system bandwidth, which may bedecided by the user equipment. The number of antenna ports used for CRStransmissions is configured by the network and is communicated to userequipments as a part of the system information broadcasted in the cell,but the user equipments expect CRS to be transmitted at least from oneantenna port, e.g. port 0.

One advantage of transmitting the CRS from multiple antenna ports is ahigher processing gain and thus more accurate measurement andpotentially a shorter measurement time. The measurement refers tomeasurements performed on CRS, e.g. Radio Resource Management (RRM)measurements, positioning measurements etc. Furthermore, CRS frommultiple antenna ports is needed for channel estimation formulti-antenna transmissions where different data streams are transmittedon different antenna ports. In the latter case, the CRS transmitted oneach multiple-antenna port needs to be different, i.e. antenna portspecific CRS.

Downlink Control Channels in LTE

Transmissions in LTE are dynamically scheduled in each subframe wherethe base station transmits assignments and/or grants to certain userequipments via a Physical Downlink Control Channel (PDCCH), which istransmitted in the first OFDM symbol(s) in each subframe and spans overthe whole system bandwidth. A user equipment that has decoded downlinkcontrol information, carried by a PDCCH, knows which resource elementsin the subframe that contain data aimed for the user equipment.

Demodulation of received data requires estimation of the radio channel,which is done by using transmitted reference symbols, i.e., symbolsknown by the receiver. For example, in LTE, cell-specific referencesymbols are transmitted in all downlink subframes and, in addition toassist downlink channel estimation, they are also used for mobilitymeasurements performed by the user equipments. LTE supports alsoUE-specific reference symbols aimed only for assisting channelestimation.

FIG. 3 illustrates an exemplary mapping of physical control and datachannels and cell specific reference signals on resource elements in adownlink subframe. In this example, the PDCCHs occupy the first 301 outof three possible OFDM symbols, and so in this particular case, themapping of data may start already at the second 303 OFDM symbol.

The length of the control region, which may vary subframe to subframe,is conveyed in the Physical Control Format Indicator Channel (PCFICH),which is transmitted within the control region, at locations known byuser equipments. After a user equipment has decoded the PCFICH, it knowsthe size of the control region and in which OFDM symbol the datatransmission starts. Also transmitted in the control region is thePhysical Hybrid-Admission Request (HARQ) Indicator Channel, whichcarries Acknowledgement/Non Acknowledgement (ACK/NACK) responses to auser equipment to inform if the uplink data transmission in a previoussubframe was successfully decoded by the base station or not.

Interference Management for RS

To ensure reliable and high-bitrate transmissions, maintaining a goodsignal quality is required in wireless networks. The signal quality isdetermined by the received signal strength and its relation to the totalinterference and noise received by the receiver. A good network plan,which among other things includes cell planning, is a prerequisite forsuccessful network operation, but it is static. For more efficient radioresource utilization, it has to be complemented at least by semi-staticand dynamic radio resource management mechanisms, which are alsointended to facilitate interference management, and deploying moreadvanced antenna technologies and algorithms.

One way to handle interference is, for example, to adopt more advancedtransceiver technologies, e.g., by implementation of interferencecancellation mechanisms in terminals. Another way, which may becomplementary to the former, is to design efficient interferencecoordination algorithms and transmission schemes in the network.

Inter-cell interference coordination (ICIC) methods for coordinatingdata transmissions between cells have been specified in LTE Release 8,where the exchange of ICIC information between cells in LTE is carriedout via an X2 interface according to a specified X2-AP protocol. The X2interface is the interface between to neighboring base stations. Basedon this information, the network can dynamically coordinate datatransmissions in different cells in the time-frequency domain and alsoby power control so that the negative impact of inter-cell interferenceis minimized.

In the current 3GPP specifications, ICIC possibilities for controlchannels are more limited. One approach of handling the interference oncontrol channels is illustrated in FIG. 4, where an interfering cell,e.g., a macro cell, does not transmit PDCCHs, and thus no data, in somesubframes 401, although other control channels may still be transmitted.The other cells, e.g., pico cells, are aware of the locations of theselow interference subframes 401 in time and can prioritize scheduling inthose subframes the user equipments which otherwise potentially maystrongly suffer from the interference caused by the interfering cell.From the legacy terminal point of view, CRS still need to be transmittedin all subframes, so there will be inter-cell interference from CRS. InFIG. 4, a thin box illustrates the control region, and the broad boxillustrates the data region. A sub frame 401 comprises one controlregion and one data region.

Given more flexibility, many techniques exist for managing interferenceto and from data channels, e.g., various time-division andfrequency-division multiplexing schemes. The possibilities toefficiently mitigate inter-cell interference to and from controlchannels are limited with the current standard. Some examples areinterleaving, time shifting, and blanking. Even less flexibility existsfor dealing with interference to and from physical signals whichtypically have a pre-defined static resource allocation in thetime-frequency space. An example of a physical signal is a referencesignal.

Some techniques for mitigating inter-cell interference known from theprior-art are:

-   -   Signal cancellation, by which the channel is measured and used        to restore the signal from, a limited number of, the strongest        interferers. Impact on the receiver implementation and its        complexity; in practice channel estimation puts a limit on how        much of the signal energy that can be subtracted.    -   Symbol-level time shifting. No impact on the standard, but not        relevant for Time Division Duplex (TDD) networks and networks        providing the MBMS service.    -   Complete signal muting in a subframe, e.g. not transmitting CRS        in some subframes for energy efficiency reasons proposed earlier        in 3GPP. Non-backward compatible to Rel.8/9 user equipments        which expect CRS to be transmitted at least on antenna port 0.

Given the very limited set of possibilities listed above, there is astrong need for simple but efficient new techniques to resolve the CRSinterference issue.

Indication of the Number of Antenna Ports

There exist techniques to allow a terminal to blindly detect the numberof antenna ports, but such techniques increase the terminal complexityand since they are in general not required by the standard they may benot implemented in the terminals.

The number of antenna ports may be signaled by the network to the userequipment as a part of the system information, e.g., as a part of theradio resource configuration information, e.g., in theAntennaInfoDedicated or AntennaInfoCommon information elements, that iscommon for all user equipments and is optionally comprised in the SystemInformation Block Type 2 (SIB2). Transmission of SIB2 is dynamicallyscheduled by the network and the scheduling information is transmittedto the user equipment as a part of System Information Block Type 1(SIB1), which is transmitted with a fixed periodicity of 80 ms in aRadio Resource Control (RRC) message via the broadcast channel andrepeated within 80 ms. There is a possibility to transmit the mostessential system information, e.g. system bandwidth, PHICH configurationor system frame number, more frequently, for which Master InformationBlock (MIB) is specified which is transmitted with a fixed periodicityof 40 ms over the broadcast channel and repeats within 40 ms, but MIBdoes not contain the information on antenna ports.

The presence of antenna port 1 can also be indicated by thePresenceAntennaPort1 information element which is a part of anEvolved-Universal Mobile Telecommunications System Terrestrial RadioAccess Network (E-UTRAN) measurement object transmitted in anRRCConnectionReconfiguration message. When PresenceAntennaPort1 is setto TRUE, the user equipment may assume that at least two cell-specificantenna ports are used in all neighboring cells.

Since they are always transmitted, CRS are a permanent source ofinterference to neighbor cells. Furthermore, when more than one antennaport is used for CRS in a cell, the CRS may be transmitted at a powerlevel higher than the reference power level utilizing the free powerfrom the unused CRS resource elements to be transmitted from anotherantenna port in the same symbol. The data can be transmitted in othersymbols than CRS symbols; the control channels have less flexibility andthus the probability of colliding with other-cell CRS is higher. For CRSmeasurements, the situation is the worst in synchronized networks, wherethe same symbols, according to the CRS transmission pattern such asexemplified in FIG. 2, are used for CRS transmissions in all cells andthese symbols with always-transmitted CRS always collide. Inasynchronous network, in general the interference on CRS is morerandomized; however, it may also happen that a CRS symbol collides witha symbol where a synchronization signal, e.g., Primary SynchronizationSignal (PSS) or Secondary Synchronization Signal (SSS), or a broadcastsignal is transmitted, which may degrade the measurement quality ofthose signals compared to if they were colliding with data symbols in alow loaded network.

Furthermore, although a cyclic prefix is used in LTE in order to maketransmissions in neighbor symbols orthogonal, it may be so that theorthogonality is not maintained between the symbols even with carefullydesigned patterns orthogonal among cells when the delay spread exceedthe cyclic prefix, which may happen in large cells or in cells inchallenging urban environments. There exist techniques for inter-symbolinterference cancellation, but the advanced techniques may significantlyincrease the user equipment complexity. This means that it is preferableto reduce the number of REs permanently allocated for transmissions,especially when such REs are the sources of high interference.

The interference generated by CRS becomes particularly crucial inheterogeneous network deployments where the transmit power maysignificantly vary by cell, e.g., a macro cell can be transmitting at 46dBm and a pico cell can be transmitting at 24 dBm, further increasingthe gap between the received interference and the received measuredsignal power. Thus the necessity of dealing with interference frommacro-cell CRS when measuring a signal from a lower-power node has beenindicated by many companies in 3GPP.

Since CRS are transmitted across a subframe, they interferer to thecontrol channels, data channels and physical signals, e.g., CRS, asdescribed above. The impact may be of a different significance in eachcase, but in general managing the CRS interference is important forimproving the overall system performance.

The existing signaling is not dynamic and flexible enough to allow fordynamic switching of antenna ports when, for example, low interferencesubframes or almost blank subframes are configured in the network.

SUMMARY

Embodiments of this invention avoid at least one of the abovedisadvantages and enjoy improved interference management incommunication networks.

According to a first aspect, the objective is achieved by a method in abase station for enabling interference coordination in a communicationnetwork. The base station comprises a plurality of antenna ports. Eachantenna port is configured to transmit a reference signal. Each antennaport is associated with a respective cell. The base station determines aset of cells where transmissions of reference signals is to be performedfrom a reduced set of the plurality of antenna ports. The base stationdetermines a subset of antenna ports in at least one cell of thedetermined set of cells to enable interference coordination in thenetwork. The base station transmits the reference signal from the subsetof antenna ports.

According to a second aspect, the objective is achieved by a basestation for enabling interference coordination in a communicationnetwork. The base station comprises a plurality of antenna ports. Eachantenna port is configured to transmit a reference signal. Each antennaport is associated with a respective cell. the base station furthercomprises a processor configured to determine a set of cells wheretransmissions of reference signals is to be performed from a reduced setof the plurality of antenna ports. The processor is further configuredto determine a subset of antenna ports in at least one cell of thedetermined set of cells, to enable interference coordination in thecommunication network. The base station further comprises a transmitterconfigured to transmit the reference signal from the subset of antennaports.

According to a third aspect, the objective is achieved by a method in auser equipment. The user equipment determines whether a reference signalis to be received from a subset of antenna ports associated with lowinterference subframes. The subset of antenna ports being comprised in abase station. The subset of antenna ports is associated with at leastone cell. The user equipment receives the reference signal from thesubset of antenna ports.

According to a fourth aspect, the objective is achieved by a userequipment in a communication network. The user equipment comprises aprocessor which is configured to determine whether a reference signal isto be received from a subset of antenna ports associated with lowinterference subframes. The subset of antenna ports is comprised in abase station. The subset of antenna ports being associated with at leastone cell. The processor is further configured to receive the referencesignal from a subset of antenna ports.

According to a fifth aspect, the objective is achieved by a method in auser equipment. The user equipment is comprised in a communicationnetwork. The method performs assisted user equipment measurementprocessing. The user equipment is comprised in a cell of a plurality ofcells in a communication network. The user equipment acquiresinformation about a set of interfering cells among the plurality ofcells. The user equipment identifies a set of time-frequency resourcesaffected by the set of interfering cells, and performs puncturing on theidentified time-frequency resources.

According to a sixth aspect, the objective is achieved by a userequipment. The user equipment is associated with a cell of a pluralityof cells in a communication network. The user equipment comprises aprocessor configured to acquire information about a set of interferingcells among the plurality of cells. The processor is further configuredto identify a set of time-frequency resources affected by the set ofinterfering cells, and to perform puncturing on the identifiedtime-frequency resources.

According to a seventh aspect, the objective is achieved by a method ina network node for enabling interference coordination in a communicationnetwork. The network node is associated with a cell. The network nodecomprising information about a set of interfering cells. The networknode acquires information about a set of interfering cells among aplurality of cells. The network node transmits the information about theset of interfering cells among a plurality of cells to a user equipmentenabling interference coordination in the communication network.

According to an eight aspect, the objective is achieved by a networknode for enabling interference coordination in a communication network.The network node is associated with a cell. The network node comprisesinformation about a set of interfering cells. The network node comprisesa processor which is configured to acquire information about a set ofinterfering cells among a plurality of cells. The network node comprisesone or more antennas configured to transmit information about the set ofinterfering cells among a plurality of cells to a user equipment,enabling interference coordination in the communication network. Thetransmitted information is based on the acquired information.

According to a ninth aspect, the objective is achieved by a method innetwork node for enabling interference coordination in a communicationnetwork. This provides enhanced cell planning adopted for heterogeneousnetwork deployments. The network node comprises a plurality of antennaports. Each antenna port is configured to transmit a reference signalaccording to a signal pattern. The network node decides an active set ofantenna ports from the plurality of antenna ports based on a reservedsubset of signal patterns associated with at least one layer of networknodes. The reserved subset of signal patterns associated with lowinterference subframes and reserved from a plurality of signal patternsor indications to signal patterns. The network node transmits referencesignals from the decided active set of antenna ports according to thereserved subset of signal pattern, enabling interference coordination inthe communication network.

According to a tenth aspect, the objective is achieved by a network nodefor enabling interference coordination in a communication network. Thenetwork node comprises a plurality of antenna ports. Each antenna portis configured to transmit a reference signal according to a signalpattern. The network node comprises a processor which is configured todecide an active set of antenna ports from the plurality of antennaports based on a reserved subset of signal patterns associated with atleast one layer of network node. The subset of signal patternsassociated with low interference subframes and reserved from a pluralityof signal patterns or indications to signal patterns. The network nodefurther comprises a transmitter configured to transmit reference signalsfrom the decided active set of antenna ports according to the reservedsubset of signal pattern, enabling interference coordination in thecommunication network.

The embodiments herein afford many advantages, for which anon-exhaustive list of examples follows:

Reduced CRS interference in the control region, on CRS, and datachannels leads to improved system performance and in particular inheterogeneous deployments.

Another advantage is that facilitating user equipment measurements withsome of the disclosed methods according to the present solution byintroducing the new signalling reduces the user equipment complexity.

Further, the embodiments herein provides the advantage of reducing theover-estimation of the radio channel quality for legacy macro userequipments, which may comprise low-interference subframes in theinterference measurements, although they will only be scheduled insubframes with potentially much higher interference.

Enhanced cell planning and interference coordination improves theperformance of heterogeneous deployments.

One advantage of transmitting the CRS from multiple antenna ports is ahigher processing gain and thus more accurate measurement andpotentially a shorter measurement time. Furthermore, CRS from multipleantenna ports is needed for channel estimation for multi-antennatransmissions where different data streams are transmitted on differentantenna ports. In the latter case, the CRS transmitted on eachmultiple-antenna port needs to be different, i.e. antenna port specificCRS.

The embodiments herein are not limited to the features and advantagesmentioned above. A person skilled in the art will recognize additionalfeatures and advantages upon reading the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The solution will now be further described in more detail in thefollowing detailed description by reference to the appended drawingsillustrating embodiments of the embodiments herein and in which:

FIG. 1 is a schematic diagram illustrating an exemplary LTE time-domainstructure.

FIG. 2 a-c are schematic diagrams illustrating an exemplary resourceelement allocation of CRS in LTE within one subframe.

FIG. 3 is a schematic diagram illustrating mapping of physicalcontrol/data channels and signals on resource elements in a downlinksubframe.

FIG. 4 is a schematic diagram illustrating ICIC using low interferencesubframes in downlink.

FIG. 5 is a block diagram illustrating embodiments of a communicationnetwork.

FIG. 6 is a combined signaling diagram and flowchart depictedembodiments of a method.

FIG. 7 a-c are schematic diagrams illustrating an example of a reducedset of active antenna ports.

FIG. 8 is a flow chart illustrating embodiments of a method in a basestation.

FIG. 9 is a block diagram illustrating an embodiment of a base station.

FIG. 10 is a flow chart illustrating embodiments of a method in a userequipment.

FIG. 11 is a block diagram illustrating an embodiment of a userequipment.

FIGS. 12&13 are flow charts illustrating embodiments of a method in auser equipment.

FIG. 14 is a flow chart illustrating embodiments of a method in anetwork node.

FIG. 15 is a block diagram illustrating an embodiment of a network node.

FIG. 16 is a flow chart illustrating embodiments of a method in anetwork node.

FIG. 17 is a block diagram illustrating an embodiment of a network node.

FIG. 18 is a block diagram illustrating an embodiment of a transmitter.

FIG. 19 is a block diagram illustrating an embodiment of a userequipment arrangement.

The drawings are not necessarily to scale, and emphasis is instead beingplaced upon illustrating the principles of the solution.

DETAILED DESCRIPTION

The embodiments herein relate to methods and apparatus which areconfigured for one or more of the following:

-   -   To facilitate control of the set of active antenna ports used        for physical signal transmissions in order to reduce the        physical signal interference,    -   For assisted user equipment measurement processing, and    -   For enhanced cell planning adopted for heterogeneous network        deployments.

The three parts may be viewed as separate embodiments or may form anycombination.

FIG. 5 depicts an embodiment of a communications network 500. Thecommunications network 500 may use technologies such as LTE, WiMAX etc.In a network 500, a mixture of cells of differently sized andoverlapping coverage areas may be deployed. A cell is a geographicalarea where radio coverage is provided by a base station. For example,the network 500 may comprise a pico cell 501 a deployed within thecoverage area of a macro cell 501 b. The pico cell 501 a may beassociated with a pico base station 503 a. The pico base station 503 aserves the pico cell 501 a. The macro cell 501 b may be associated witha macro base station 503 b. The macro base station 503 b serves themacro cell 501 b. In the following description, the reference number 501will be used for indicating a cell in general, and the reference number503 will be used for indicating a base station in general. The basestation 503 may be e.g. a pico base station, a macro base station, HomeBase Station (HBS), radio base station, e nodeB (eNB), base station,relay, remote radio heads etc, or any other network unit capable tocommunicate over a radio carrier with a user equipment 505. The userequipment 505 may be present within the cell 501 and served by the basestation 503. More than one cell can be associated with one base station.As a network 500 may comprises a plurality of nodes, a base station may,in some embodiments, be called a network node. A base station 503comprises at least one antenna port (not shown), e.g. antenna port 0.Each antenna port is configured to transmit and receive signals from thebase station 503 to e.g. one or more user equipment 505. In other words,each antenna ports comprise receivers and transmitters. Other examplesof network nodes are, for instance, positioning nodes, Operations &Maintenance (O&M) nodes etc.

A downlink (DL) is the link from a base station 503 down to one or moreuser equipments 505, and an uplink (UL) is the link from a userequipment 505 up to a base station 503. A user equipment 505 comprisedin the network 500 is assigned to a certain cell, which is referred toas the serving cell.

In the following, the user equipment 505 comprises for example, mobiletelephones, pagers, headsets, laptop computers and other mobileterminals, and the like. In a broader sense, user equipment 505 may alsobe understood as a general wireless device or any device equipped with aradio interface and even small base stations capable of receivingsignals in downlink, sensors, relays, etc. fall into this category andthus covered by the current invention.

Dynamic Control of the Set of Active Antenna Ports for Physical SignalTransmissions

For backward-compatibility reasons, CRS cannot be turned off completelyin a subframe 103,401 as illustrated in FIGS. 1 and 4. For example, the3GPP standard requires that for RSRP determination, the CRS R0, i.e. CRSon antenna port 0, shall be used, meaning that CRS has to always betransmitted at least from antenna port 0. If the user equipment 505 canreliably detect that R1, i.e. CRS on antenna port 1, is available, theuser equipment 505 can use R1 in addition to R0 to determine RSRP. Somemethods exist for signaling the antenna information, but they are notdynamic and flexible enough to support the operation of heterogeneousnetwork as explained above.

In accordance with embodiments herein, the set of active antenna portsmay be activated/deactivated dynamically to control the RS interference.In a specific example, a reduced set of active antenna ports isassociated with low-interference subframes, which are used to improveperformance of some user equipments, to minimize or avoid RSinterference from the strongly interfering cells. The stronglyinterfering cells may be defined by their absolute or relative, e.g.with respect to the serving cell, signal strength. The cells or basestations may also be sometimes classified as strong interferers whenthey are associated to base station 503 of a higher power class, e.g.macro cells may be viewed in this way as stronger interferers comparedto pico cells

Low-interference subframes, seen from the perspective of the userequipment 505, imply a reduced level of received interference. A reducedlevel of interference may be achieved by e.g. scheduling less userequipments on data channels. A similar effect may be achieved byconfiguring positioning subframes or empty MultimediaBroadcast/Multicast Service (MBSFN) Single Frequency Network (MBSFN)subframes, without transmitting broadcast data. Further, theinterference in the network is improved by including the timescorresponding to such subframes. The reduced level of received interfacemay also be achieved by using Almost Blank Subframes (ABS). ABS may bedefined as subframes with reduced transmission power and/or activity.Low-interference subframes may be associated with a time with specificinterference conditions.

In relation to antenna ports associated with low-interference subframes,it should be noted this refers to antenna ports seen by the receiverside that face different interference. Thus, the transmitter does notface any interference.

Embodiments of a suitable method will now be described with reference tothe combined signaling diagram and flowchart depicted in FIG. 6 and withreference to FIG. 5 illustrating embodiments of a communication network500. The method comprises at least some of the following steps, whichsteps may as well be carried out in another suitable order thandescribed below.

Step 601

The base station 503 determines the time when the RS transmissions haveto be performed from fewer antenna ports, i.e. when reduced or lowinterference is desired. The time is associated with low-interference,i.e. low-interference subframes. A subframe may represent a timeinterval or time period

Step 602

The base station 503 determines the set of cells where the reduced setof antenna ports shall apply.

Step 603

The base station 503 determines the reduced set of antenna ports in atleast one cell from the set determined in step 602. The fewer antennaports may comprise a subset of an original set of antenna ports.

Step 604

In some embodiments, the base station 503 informs the user equipment 505about a temporary change of the set of active antenna ports and(optionally) about a time interval during which the reduced set ofantenna ports shall apply. In other words, the base station 503 may ormay not inform the user equipment 505 about the time interval.

Step 605

The base station 503 transmits the RS from the reduced set of antennaports.

Step 606

In some embodiments, the user equipment 505 performs measurements andreporting in the reduced set of antenna ports.

Step 607

The base station 503 re-initiates or restores the RS transmissions fromthe original set of antenna ports.

Step 608

The base station 503 informs the user equipment 505 about the restoredRS transmissions.

Those steps need not to be carried out in the exact order listed aboveand some steps may be omitted. The steps are described in more detailbelow, and each step description corresponds to a respective set ofseparate embodiments. The embodiments may also be combined.

Step 601: Determining the Time when the RS Transmissions have to bePerformed from a Fewer Antenna Ports

The switch time to a reduced set of active antenna ports, i.e. theswitch time to the temporary change, may occur in accordance to asignaled pattern, or periodically or by a trigger.

A signaled pattern may be the same as the pattern of low-interferencesubframes or almost blank subframes intended to improve the interferencesituation for user equipment 505 that may otherwise potentially havepoor performance.

A trigger for the temporary change may be based, for example, on adetermined indication that the interference from a certain cell, e.g.,cell 1, causes unacceptable performance degradation in some area ofanother cell, e.g., cell 2. The indication may be deduced from ameasurement, such as signal quality measurements in cell 2 in that area,and where the indication may be communicated by cell 2 to cell 1 via theX2 interface.

In one embodiment, the indication in cell 1 is received from a networknode, e.g., an Operations & Maintenance (O&M) node (not shown), whichcollects different measurements from different cells. In anotherembodiment, the indication in cell 1 is deduced by cell 1 itself basedon the available measurements.

Step 602: Determining the Set of Cells where the Reduced Set of AntennaPorts Shall Apply

Below are possible options for deciding the cells where the set ofactive antenna ports may be reduced, i.e. the temporary change of activeantenna ports:

-   -   a. The set of active antenna ports may be changed in all cells        in the network 500, or    -   b. The set of active antenna ports may be changed in all macro        cells 105, or    -   c. The set of active antenna ports may be changed in cells with        given overlapping RS patterns, e.g., corresponding to a certain        frequency shift, or    -   d. The set of active antenna ports may be changed in macro cells        with RS patterns overlapping with the RS pattern of a        lower-power node in its proximity, or    -   e. The reduced set of active antenna ports may be pre-configured        by the operator in the selected cells or configured by O&M.        Step 603: Determining the Reduced Set of Antenna Ports in at        Least One Cell in the Network

In an example, the number of CRS antenna ports is reduced from 2 orlarger to 1 antenna port, which means increasing the effective reusefactor, or non-overlapping frequency shifts, from 3 to 6. The set ofactive antenna ports is configured to avoid interference from at leastone strong interferer. In one example, macro cells may be considered asstrong interferers compared to pico cells. In another example, CSG femtocells may be considered as strong interferers e.g. compared to pico ormacro cells.

In one embodiment, antenna port 0 shall always be included in the set ofactive antenna ports e.g. when the reference signals are CRS and CRStransmissions are required from at least the antenna port 0, but thismay be not necessarily in other embodiments of the present solution.

In another embodiment, the set of active antenna ports in one layer ofnodes, e.g., macro layer, is chosen to avoid the overlap with patternsreserved for another layer, e.g., pico node. More details on thereserved patterns are provided below.

In yet another embodiment, the set of active antenna ports is decideddepending on the CRS transmission pattern and/or the set of activeantenna ports in the interfering neighbor cell, the information on theactive set of antenna ports may be exchanged among the neighbor cellsover the X2 interface.

Step 604: Informing the UE About the Temporary Change of the Set ofActive Antenna Ports and (Optionally) about the Time Periods Duringwhich the Reduced Set of Antenna Ports Apply

At least two ways of acquiring this information by the user equipment505 are envisioned: the information is pre-determined and known to theuser equipment 505(a) or it is signaled by the network to the userequipment 505(b).

(a) The pre-determined information may comprise:

-   -   The reduced set of antenna ports.    -   The periodicity of time intervals when the reduced set of        antenna ports applies.    -   The consecutive time interval when the reduced set of antenna        ports applies.    -   The configured bandwidth where the reduced set of active antenna        ports applies.    -   An indication whether it applies to the control region only.

(b) Information signaled to the user equipment 505:

-   -   An indication that a pre-defined reduced set may be used during        a pre-defined interval with a pre-defined periodicity, or    -   At least some part of the information described in 604(a)

For example, only the number of antenna ports in the reduced set may besignaled, if desired. The signaled information may be by nature userequipment-specific, e.g. user equipments 505 in a challenging area, orcell-specific and thus broadcast, e.g., via one or more suitableinformation elements in one or more suitable SIBs.

(c) In another embodiment, the user equipment behavior is such that theuser equipment 505 may assume that the pre-defined, pre-configured orthe signaled reduced set configuration applies starting with thelow-interference or almost blank subframes about which the userequipment 505 has the information. Some examples of such information maybe the received by the user equipment Almost Blank Subframes (ABS)pattern(s), defined as subframes with reduced transmission power and/oractivity, and a measurement pattern signaled over RRC by the servingbase station. Another example is a positioning subframe configurationsignaled by the network to facilitate positioning. The information aboutthe set of active antenna ports in such subframes may thus be signaledtogether with the low-interference subframe or almost blank subframeconfiguration.

Step 605: Transmitting the RS from the Reduced Set of Antenna Ports

-   -   (a) The transmitting the RS from the reduced set of antenna        ports may be periodic.    -   (b) The pre-defined or pre-configured or dynamically configured        reduced set of active antenna ports in a cell is invoked by an        event, e.g., triggering together low-interference subframes.        Step 606: Measuring and Reporting in the Reduced Set of Antenna        Ports

Some user equipments 505 may conduct some measurements only during thetime when the RS are transmitted from the reduced set of antenna portsand the other user equipments (not shown) may not use these subframesfor measurements, e.g., when these user equipments 505 are scheduled insuch subframes with a low probability. The conducted measurements mayalso be reported to the network or be used internally in the userequipment 505. Such measurement coordination may be an advantage, forexample, when high interference is expected in some subframes so thatsome user equipments 505 connected to pico cells 501 a may not able toperform measurements in the se subframes.

Step 607: Restoring the RS Transmissions from the Original Set ofAntenna Ports in the Cell

-   -   (a) The restoring may be performed by a stop-trigger, or    -   (b) The restoring may be performed after the configured interval        is over, or    -   (c) The restoring may be performed associated with the end of        low-interference subframes.        Step 608: Informing the User Equipment 505 about the Restored RS        Transmissions    -   (a) The user equipment 505 behavior may be such that the user        equipment 505 may assume that the cell switches to the original        antenna port configuration for RS in the end of low-interference        subframes so the decision is made by the user equipment 505        autonomously, or    -   (b) An indicator may be sent to user equipments 505, e.g., by        broadcast via a suitable SIB in the cell, that the original set        of antenna ports will be restored.        Time-Frequency Resources where the Reduced Set of Active Antenna        Ports can Apply:

FIGS. 7 a-c illustrates an example of a reduced set of active antennaports. The hatched regions illustrate a control region. The squaresillustrate CRS reference signals for antenna port 0 and the circlesillustrates CRS reference signal for antenna port 1. The reduced set ofactive antenna ports apply in the following scenarios:

-   -   (1) Within the entire resource block, one subframe in time, over        the system bandwidth or a configured bandwidth, which may be        smaller than the system bandwidth, as illustrated in FIG. 7 a.        or    -   (2) Within the control region of the subframe over the system        bandwidth or a configured bandwidth, which may be smaller than        the system bandwidth, as illustrated in FIG. 7 b or    -   (3) Within a subset of subcarriers and/or a subset of symbols of        each resource block within a given subframe and over the system        bandwidth or a configured bandwidth, which can be smaller than        the system bandwidth, as illustrated in FIG. 7 c.    -   (a) An example of using fewer transmit antenna ports in a part        of the subframe is when that part collides with, for example,        synchronization signals in other cells in a asynchronous        network, where such blanking can be pre-determined for a given        synchronization requirement which would in turn also        pre-determine the user equipment measurement behavior.    -   (b) In one embodiment (not compliant with Release 8), the active        set of antenna ports is chosen based on the number of allowed        for transmission subcarriers.        RS Transmit Power

With more than one active antenna ports, a cell has a possibility toboost the CRS power by 3 dB by just reusing the power from the resourceelements, also referred to as REs, where another CRS is transmitted fromanother antenna. By configuring one antenna port, the CRS Energy PerResource Element (EPRE) in the cell is more likely to be at the levelassuming the constant EPRE across the transmission bandwidth, which maybe viewed as a way to control the CRS EPRE and thus keep the CRSinterference from the given cell at a lower level.

The method described above will now be described seen from theperspective of the base station 503. FIG. 8 is a flowchart describingthe present method in the base station 503 for enabling interferencecoordination in a communication network 500. The base station 503comprises a plurality of antenna ports. Each antenna port is configuredto transmit a reference signal. The reference signals are notspecifically transmitted to any user equipment 505, even though the userequipment 505 may receive some assistance in other scenarios. Thesignaling of the reference signal to a user equipment is not dedicatedsignal. The reference signal may be received by a plurality of userequipments 505. Each antenna port is associated with a respective cell101, 105. In some embodiments, a plurality of antenna ports isassociated with each respective cell 101, 105. In some embodiments, theinterference coordination is implemented with respect to a highinterference area of a cell. The method comprises the steps to beperformed by the base station 503:

Step 801

This step corresponds to step 601 in FIG. 6. In some embodiments, thebase station 503 determines a time when the reference signal is to betransmitted from a reduced set of antenna ports. The time is associatedwith low interference, i.e. low interference subframes. The referencesignal is transmitted from the subset of the antenna ports to the userequipment 505 at the determined time.

Step 802

In some embodiments, the base station 503 informs the user equipment 505about the determined time.

Step 803

This step corresponds to step 602 in FIG. 6. The base station 503determines a set of cells 101, 105 where transmissions of referencesignals is to be performed from a reduced set of the plurality ofantenna ports.

In some embodiments, the determined subset of antenna ports isconfigured to avoid interference from an interfering cell 101, 105 orreduce interference to another cell 501.

Step 804

This step corresponds to step 603 in FIG. 6. The base station 503determines a subset of antenna ports in at least one cell 501 of thedetermined set of cells 101, 105. The subset of antenna ports isassociated with low interference, i.e. low interference subframes.

In some embodiments, the subset of antenna ports is pre-configured.

In some embodiments, at least one of: the determined time andinformation of the subset of antenna ports, is obtained from a networknode (not shown) in the communication network (500). The network nodemay be a base station different from the base station 503, i.e. via X2.The network node may be e.g. a radio network node (BS) or anothernetwork node such as O&M node.

Step 805

This step corresponds to step 604 in FIG. 6. In some embodiments, thebase station 503 informs the user equipment 505 about the subset ofantenna ports.

Step 806

This step corresponds to step 604 in FIG. 6. In some embodiments, thebase station 503 determines a time interval during which the subset setof antenna ports shall apply.

Step 807

This step corresponds to step 604 in FIG. 6. In some embodiments, thebase station 503 informs the user equipment 505 about the time interval.

Step 808

This step corresponds to step 605 in FIG. 6. The base station 503transmits the reference signal from the subset of antenna portsassociated with low interference, i.e. low interference subframes,enabling interference coordination in the communication network 500.

In some embodiments, the reference signal is transmitted to the userequipment 505.

In some embodiments, the transmissions from the reduced set of antennaports apply to a part of system bandwidth.

In some embodiments, the transmissions from the reduced set of antennaports in a cell 101, 105 are periodic or invoked by an event.

In some embodiments, the signaling from the base station 503 is notdedicated to a specific user equipment 505, but may be transmitted to aplurality of user equipments 505 in the communication network 500, e.g.,the signaling may be cell-specific and transmitted over the cell area,and thus potentially may be used by any user equipment 505 performingmeasurements on that cell.

In some embodiments, the signaling from the base station 503 isdedicated to a specific user equipment 505.

Step 809

This step corresponds to step 607 in FIG. 6. In some embodiments, thebase station 503 re-initiates the reference signal transmissions fromthe plurality of antenna ports.

Step 810

This step corresponds to step 607 in FIG. 6. In some embodiments, thebase station 503 informs the user equipment 505 about the re-initiatedreference signal transmissions.

Step 811

This step corresponds to step 606 in FIG. 6. In some embodiments, thebase station 503 receives measurements from the user equipment 505.

To perform the method steps shown in FIG. 8 for enabling interferencecoordination in the communication network 500. The base station 503comprises a base station arrangement as shown in FIG. 9. The basestation 503 comprises a plurality of antenna ports. Each antenna port isconfigured to transmit a reference signal. Each antenna port isassociated with a respective cell 101, 105. In some embodiments, theinterference coordination is implemented with respect to a highinterference area of a cell.

The base station 503 further comprises a processor 901 which isconfigured to determine a set of cells 101, 105 where transmissions ofreference signals is to be performed from a reduced set of the pluralityof antenna ports. The processor 901 is further configured to determine asubset of antenna ports in at least one cell 101, 105 of the determinedset of cells 101, 105. In some embodiments, the determined subset ofantenna ports is configured to avoid interference from an interferingcell 101, 105 or from another cell 501. In some embodiments, the subsetof antenna ports is pre-configured.

The base station 503 further comprises a transmitter 1800 configured totransmit the reference signal from the subset of antenna portsassociated with low interference, i.e. low interference subframes,enabling interference coordination in the communication network 500. Thetransmitter 1800 is described in more detail in relation to FIG. 18below. In some embodiments, the transmissions from the reduced set ofantenna ports apply to a part of system bandwidth. And, in someembodiments the transmissions from the reduced set of antenna ports in acell 101, 105 are periodic or invoked by an event.

In some embodiments, the processor 901 is further configured todetermine a time when the reference signal is to be transmitted from areduced set of antenna ports. The time is associated with lowinterference, i.e. low interference subframes. The reference signal istransmitted from the subset of the antenna ports at the determined time.In some embodiments, the processor 901 is further configured to informthe user equipment 505 about the determined time, and to inform the userequipment 505 about the subset of antenna ports. In some embodiments, atleast one of the determined time and information of the subset ofantenna ports, is obtained from a network node.

In some embodiments, the processor 901 is configured to determine a timeinterval during which the subset set of antenna ports shall apply, andto inform the user equipment 505 about the time interval.

In some embodiments, the processor 901 is further configured tore-initiate the reference signal transmissions from the plurality ofantenna ports, and to inform the user equipment 505 about there-initiated reference signal transmissions.

In some embodiments, the processor 901 is configured to receivemeasurements from the user equipment 505.

The method described above will now be described seen from theperspective of the user equipment 505. FIG. 10 is a flowchart describingthe present method in the user equipment 505. The method comprises thefurther steps to be performed by the user equipment 505:

Step 1001

This step corresponds to step 601 in FIG. 6. In some embodiments, theuser equipment 505 receives information from the base station 503 abouta time. The time indicates when the reference signal is to be receivedfrom a subset of antenna ports. The time is associated with lowinterference, i.e. low interference subframes.

Step 1002

This step corresponds to step 604 in FIG. 6. In some embodiments, theuser equipment 505 receives information from the base station 503 aboutthe subset of antenna ports.

In some embodiments, the subset of antenna ports is pre-defined.

In some embodiments, the subset of antenna ports is pre-defined for alayer of nodes.

Step 1003

This step corresponds to step 604 in FIG. 6. In some embodiments, theuser equipment 505 receives information from the base station 503 abouta time interval. The time interval indicates a time period during whichthe subset set of antenna ports shall apply.

In some embodiments, the information about the time interval comprisesan indication on whether a subset of antenna ports applies or not in thetime associated with low interference subframes, i.e. specificinterference conditions, e.g. when only pico cells are transmitting andthus the expected interference is only from pico cells.

Step 1004

The user equipment 505 determines whether a reference signal is to bereceived during specific interference conditions. In other words, theuser equipment 505 determines whether a reference signal is to bereceived from a subset of antenna ports associated with lowinterference, i.e. low interference subframes. The subset of antennaports is comprised in a base station 503. The subset of antenna ports isassociated with at least one cell 501.

In some embodiments, the determining whether a reference signal is to bereceived from a subset of antenna ports is based on at least one of areference signal pattern and a set of active antenna ports in aninterfering neighbour cell 501. The antenna port information isexchanged over the X2 interface. The set of active antenna ports isdetermined depending on the CRS transmission pattern and/or the set ofactive antenna ports in the interfering neighbour cell. In other words,the information on the active set of antenna ports can be exchangedamong the neighbour cells over the X2 interface.

Step 1005

This step corresponds to step 605 in FIG. 6. The user equipment 505receives a reference signal from the subset of antenna ports. The subsetof antenna ports is comprised in a base station 503.

In some embodiments, the reference signal is received from the subset ofthe antenna ports at the time.

Step 1006

This step corresponds to step 607 in FIG. 6. In some embodiments, theuser equipment 505 receives information from the base station 503 aboutre-initiated reference signal transmissions.

Step 1007

This step corresponds to step 606 in FIG. 6. In some embodiments, theuser equipment 505 performs measurements on the subset of antenna ports.

Step 1008

This step corresponds to step 606 in FIG. 6. In some embodiments, theuser equipment 505 transmits the measurements to the base station 503.

To perform the method steps shown in FIG. 10 the user equipment 505comprises a user equipment arrangement as shown in FIG. 11 and FIG. 19.The user equipment 505 comprises a processor 1916 which is configured toreceive a reference signal from a subset of antenna ports. The subset ofantenna ports is comprised in a base station 503.

In some embodiments, the subset of antenna ports is pre-defined, and insome embodiments the subset of antenna ports is pre-defined for a layerof nodes. The processor 1916 is further configured to determine whetherthe reference signal is to be received from a subset of antenna portsbeing comprised in a base station 503. The subset of antenna ports isassociated with at least one cell 501. The user equipment arrangement isfurther described in relation to FIG. 19 below.

In some embodiments, the processor 1916 is further configured to receiveinformation from the base station 503 about a time. The time indicateswhen the reference signal is to be received from a subset of antennaports, and the time is associated with low interference, i.e. lowinterference subframes. In some embodiments, the reference signal isreceived from the subset of the antenna ports at the time.

In some embodiments, the processor 1916 is further configured to receiveinformation from the base station 503 about the subset of antenna ports.

In some embodiments, the processor 1916 is further configured to receiveinformation from the base station 503 about a time interval. The timeinterval indicates a time period during which the subset set of antennaports shall apply. In some embodiments, the information about the timeinterval comprises an indication on whether a subset of antenna portsapplies or not in the time associated with low interference subframes,i.e. associated with specific interference conditions.

In some embodiments, the processor 1916 is further configured to receiveinformation from the base station 503 about re-initiated referencesignal transmissions.

In some embodiments, the processor 1916 is further configured to performmeasurements on the subset of antenna ports. In some embodiments, theantenna 1902 is further configured to transmit the measurements to thebase station 503.

Method and Apparatus for Assisted User Equipment Measurement Processing

The user equipment 505 receives the assistance information from thenetwork about the strongest interferer(s) and, based on thisinformation, the user equipment 505 selects the desired number of themost critical interferers and use the information to improve the controlchannel decoding, CRS measurements, channel estimation, e.g., by notincluding the unreliable part of the channel information, etc.

The assistance data may comprise one or more of:

-   -   A set of PCIs (Physical Cell Identifier), based on which the        user equipment 505 can, for example, determine the RS pattern.    -   Transmit bandwidth of the interferers.    -   Channel-related information or the information based on which        the channel information may be deduced, e.g., exploit the        channel reciprocity in TDD.    -   Number of antenna ports.

The assistance information may be signaled together with theconfiguration of the low transmission activity pattern determining whenlow-interference subframes or almost blank subframes occur. Theassistance data may be tailored specifically for heterogeneous networks,e.g., includes the information for specific layer nodes, e.g., aboutonly the cells with higher transmit power than the current one or onlythe CSG cells.

The assistance data are typically transmitted to a certain userequipment 505. That user equipment 505 is connected to the network 500and is assigned to a certain cell, which is then the serving cell.

In one embodiment, the assistance data is signaled by the serving cellwhich in turn either autonomously obtains this information, e.g., basedon collected measurements or from the O&M, or receives this informationfrom another node, e.g., the interfering macro cell “identifies” itselfvia the X2-interface to the pico cells 501 a located in the range ofthat macro cell 501 b coverage.

In another embodiment, the assistance data is transmitted to the userequipment 505 to assist in its operation in the identified specificallychallenging interference conditions and may thus be triggered when sucha condition is detected. E.g. when the user equipment 505 enters aClosed Group Subscriber (CSG) cell coverage area but cannot connect tothe cell the macro cell 501 b can signal the assistance information tothe user equipment 505 which includes also the identity of the Home eNB(HeNB). The Home NodeB is the base station 505 of the CSG cell. CSG iscalled “Closed subscriber group” because even if the user equipment 505can detect a strong signal, and good signal quality, for that cell, theuser equipment 505 cannot connect to it, i.e. “Closed . . . ” Thisleaves the macro cell 501 b being the serving cell for that userequipment 505.

In yet another embodiment, the assistance data is extracted by the userequipment 505 autonomously from a special-purpose assistance datasignaled by the network 500, e.g.:

-   -   From the OTDOA positioning assistance data which the user        equipment 505 may receive being positioned or may request from        the network by sending a positioning request which may indicate        a preferred positioning method, e.g., Observed Time Difference        of Arrival (OTDOA), for which the assistance data of interest        may be expected.    -   From the mobility lists comprising at least the identities of        neighbor cells, which in most cases will be also the strongest        interferers.

Methods and apparatus for assisted user equipment measurement processingwill now be described with reference to the flowchart depicted in FIG.12. The methods and apparatus are configured to implement at least thefollowing:

Step 1201

The user equipment 505 identifies the set of strongest interferers,using one of the approaches described above.

Step 1202

The user equipment 505 decides the set of the most crucial interferers,i.e., the set may be smaller than that obtained in Step 1201 due to, forexample, user equipment 505 capability. The decision may also accountfor efficient cell grouping, and identifying deriving the set of thetime-frequency resources affected by these interferers by utilizing theknowledge of the RS transmit pattern.

Step 1203

The user equipment performs puncturing on the identified time-frequencyresources when measuring the signal in the user equipment 505 side. Inmore detail, puncturing of time-frequency resources is equivalent toexcluding the identified time-frequency resources or setting the weightson the identified time-frequency resources to zero when performingmeasurements.

The method described above for assisted user equipment measurementprocessing will now be described seen from the perspective of the userequipment 505. The method in the user equipment may be transparent tothe network, and will enable interference mitigation in thecommunication network 500. The user equipment 505 is associated with acell 501 of a plurality of cells in a communication network 500. FIG. 13is a flowchart describing the present method in the user equipment 505.The method comprises the steps to be performed by the user equipment505:

Step 1301

This step corresponds to step 1201 in FIG. 12. The user equipment 505acquires information about a set of interfering cells 501 among theplurality cells 501. The set of interfering cells are strong interferes.The acquiring of information may be performed by extracting, see step1302, or receiving the information.

In some embodiments, each cell in the set of interfering cells isassociated with a strong interfering signal, which strong interferingsignal has a signal strength above a threshold.

In some embodiments, the information about interfering cells 501comprises assistance data.

In some embodiments, the assistance data comprises at least one of a setof physical layer cell identities, a transmit bandwidth of aninterfering cell 501, channel-related information and a number ofantenna ports.

In some embodiments, the information about interfering cells 501 isacquired, i.e. received, from a serving cell or a network node (notshown) within the communication network 500.

The network node may be a radio network node and non-radio networknodes, e.g., a positioning node or other coordinating node. For the sakeof simplicity, only radio network nodes are shown in FIG. 5.

For example, OTDOA assistance data may be received from a network nodewhich is not a radio node, but for example a positioning node in thecore network. Ultimately in the physical layer, the data are oftransmitted by the radio base station over the radio link to the userequipment 505, but the information is transmitted over a higher-layerprotocol which is between the positioning node and user equipment 505and the transmitted data are then transparent to the radio base station.In another example, the information may be transmitted by the serving orother radio base station, and in this case it any radio network node503.

In some embodiments, the set of interfering cells 501 is a subset of theset of interfering cells. The subset of cells may be based on at leastone of: user equipment capability, cell grouping, a desired number ofmost critical interfering cells to account for puncturing, and impact onan interference level.

Step 1302

This step corresponds to step 1201 in FIG. 12. In some embodiments, theuser equipment 505 autonomously extracts the assistance data from theacquired information.

Step 1303

This step corresponds to step 1203 in FIG. 12. The user equipment 505identifies a set of time-frequency resources affected by the subset ofinterfering cells 501.

In some embodiments, the identifying a set of time-frequency resourcesis based on a reference signal transmit pattern.

Step 1304

This step corresponds to step 1203 in FIG. 12. The user equipment 505performs puncturing on the identified time-frequency resources. In moredetail, Puncturing of time-frequency resources is equivalent toexcluding the identified time-frequency resources or setting theidentified time-frequency resources to zero.

To perform the method steps shown in FIG. 13 for assisted user equipmentmeasurement processing, the user equipment 505 comprises a userequipment arrangement as shown in FIG. 11 and FIG. 19. The userequipment 505 is associated with a cell 501 of a plurality of cells 501in a communication network 500.

The user equipment 505 comprises a processor 1916 configured to acquire,i.e. extract or receive, information about a set of interfering cells501 among the plurality of cells 501. In some embodiments, theinformation about interfering cells 501 is received from a serving cellor a network node within the communication network 500.

In some embodiments the set of interfering cells 501 is a subset of theset of interfering cells, which subset of cells is based on at least oneof: user equipment capability, cell grouping, a desired number of mostcritical interfering cells to account for puncturing, and an impact onan interference level.

The processor 1916 is further configured to identify a set oftime-frequency resources affected by the set of interfering cells 501,and to perform puncturing on the identified time-frequency resources.

In some embodiments, the identifying a set of time-frequency resourcesis based on a reference signal transmit pattern.

In some embodiments, the assistance data comprises at least one of a setof physical layer cell identities, a transmit bandwidth of aninterfering cell 501, channel-related information and a number ofantenna ports.

In some embodiments, the processor 1916 is further configured toautonomously extracting the assistance data from the acquiredinformation.

The method described above for assisted user equipment measurementprocessing which enables interference coordination in a communicationnetwork 500 will now be described seen from the perspective of thenetwork node 503. The network node 503 is associated with a cell 501.The network node 503 comprises information about a set of interferingcells 101, 105. FIG. 14 is a flowchart describing the present method inthe network node 503. The method comprises the steps to be performed bythe network node 503:

Step 1401

This step corresponds to step 1201 in FIG. 12. The network node 503acquires information about a set of interfering cells 501 among theplurality cells 501. The set of interfering cells are strong interferes.The acquiring of information may be performed by extracting, orreceiving the information.

In some embodiments, the information about interfering cells 501comprises assistance data.

In some embodiments, the assistance data comprises at least one of a setof physical layer cell identities, a transmit bandwidth of aninterfering cell 501, channel-related information and a number ofantenna ports.

In some embodiments, the information about interfering cells 501 isacquired, i.e. received, from a serving cell or another network node(not shown) within the communication network 500.

In some embodiments, the set of interfering cells 501 is a subset of theset of interfering cells, which subset of cells is based on at least oneof: user equipment capability, cell grouping, a desired number of mostcritical interfering cells to account for puncturing, and an impact onan interference level.

In some embodiments, each cell in the set of interfering cells isassociated with a strong interfering signal, which strong interferingsignal has a signal strength above a threshold.

Step 1402

This step corresponds to step 1201 in FIG. 12. The network node 503transmits information about a set of interfering cells 501 among aplurality of cells 501 to a user equipment 505. The set of interferingcells 501 are strong interferers, enabling interference coordination inthe communication network 500.

In some embodiments, the information about interfering cells 501comprises assistance data. In some embodiments, the assistance datacomprises at least one of a set of physical layer cell identities, atransmit bandwidth of an interfering cell 501, channel-relatedinformation and a number of antenna ports.

In some embodiments, the network node 503 is associated with aninterfering macro cell 501 b.

In some embodiments, the transmitting information about the set ofinterfering cells is triggered upon detecting challenging interferenceconditions for the user equipment 505 in the communication network 500.

To perform the method steps shown in FIG. 14 for enabling interferencecoordination in a communication network 500 the network node 503comprises a network node arrangement as shown in FIG. 15. The networknode 503 is associated with a cell 501. The network node 503 comprisesinformation about a set of interfering cells 101, 105.

The network node 503 comprises a processor 1501 configured to acquireinformation about a set of interfering cells 501 among a plurality cells501.

Further, the network node comprises one or more antennas 1902 configuredto transmit 1201 the information about the set of interfering cells 501among a plurality cells 501 to a user equipment 505. The transmittedinformation is based on the acquired information. The set of interferingcells 501 are strong interferers, enabling interference coordination inthe communication network 500. In some embodiments, the informationabout interfering cells 501 comprises assistance data. In someembodiments, the network node 503 is associated with an interferingmacro cell 501 b. In some embodiments, the one or more antenna(s) 1902are further configured to transmit the information about the set ofinterfering cells to the user equipment 505 when challenginginterference conditions for the user equipment 505 are detected in thecommunication network 500. The challenging interference conditions may,for example, comprise a received signal quality level below a certainthreshold reported by the user equipment 505, radio link failurestatistics for that user equipment 505, or low signal quality expectedfor the user equipment 505 based on the network knowledge about theserving cell and interfering neighbor cells for that user equipment 505,which may also be complemented with the knowledge about the expectedrelative received signal strengths of these cells for the user equipment505.

The present mechanism for enabling interference coordination in thecommunication network 500 may be implemented through one or moreprocessors, such as a processor 1501 depicted in FIG. 15, together withcomputer program code for performing the functions of the presentsolution.

Methods and Apparatus for Enhanced Cell Planning Adopted forHeterogeneous Network Deployments

In accordance with embodiments herein, a subset of RS patterns, e.g., asubset of 6 possible frequency shifts for CRS, is reserved for at leastone layer of node, e.g., low-power nodes, and this information is usedfor deciding the active set of antenna ports. With such a reservation,the macro-cell CRS interference to CRS of low-power nodes may beavoided. The signal patterns comprise one or more pattern identities.

In one embodiment, the low-power node fetches, i.e. requests, thereserved pattern identity, or alternatively, own PCI, from the macronetwork, e.g., from O&M, which signals this information in reply. Thiscan be done by a newly installed low-power node which “joins” thenetwork 500.

In a scenario with sparsely located low-power nodes, the reserved setcomprises one RS pattern. But in general, the set of available reservedpattern identities is dynamically maintained by the network 500, e.g.,the hosting macro cell, and it depends on the pattern in use in thearea.

Using the reserved set of patterns for one layer, e.g., pico nodes, incombination with almost blank subframes, i.e. no control and/or datatransmissions, allows to completely avoid CRS interference between thelayers when the reserved patterns for one layer are orthogonal to thoseused by the other-layer nodes.

In one embodiment, the reserved set of patterns is designed accountingfor the set of active antenna ports to be used by the other layer inlow-interference or almost blank subframes. For example:

-   -   If the set of active antenna ports is empty, which may be        possible in future 3GPP Releases, for the macro layer in some        area, then all patterns may be reused by the other layer of        nodes in the same area.    -   If one antenna port is to be used by the macro layer, e.g., the        set of active antenna ports comprises one antenna port, then the        effective CRS pattern reuse is six, so one or two patterns, or        even more, depending on the low-power node density, can be        reserved for the other layer.    -   If two to four antenna ports are to be used by the macro layer,        then the effective CRS pattern reuse is three, so one pattern        could then be reserved for the other layer leaving two        orthogonal patterns to the macro layer.

It is straightforward that such a reservation scheme can be designed forany number of layers of nodes in the network.

The method described above for enhanced cell planning adopted forheterogeneous network deployments which enables interferencecoordination in a communication network 500 will now be described seenfrom the perspective of the network node 503. The network node 503comprises a plurality of antenna ports. Each antenna port is configuredto transmit a reference signal according to a signal pattern. Thetransmission of the reference signal is non-dedicated, i.e. it may bereceived by a plurality of user equipments 505. FIG. 16 is a flowchartdescribing the present method in the network node 503. The methodcomprises the steps to be performed by the network node 503:

Step 1601

In some embodiments, the network node 503 reserves a subset of signalpatterns for at least one layer of network node 503. The subset ofsignal patterns is reserved from a plurality of signal patterns orindications to signal patterns, and the subset of signal patterns isassociated with low interference subframes.

In some embodiments, subset of signal patterns is reserved for a groupof cells or a group of network nodes 503. The person skilled in the artcan recognize that a set of cells is associated with a radio networknode, and a set of cells comprises at least one cell.

In some embodiments, the group network nodes 503 belong to a layer.

In some embodiments, the reserved subset of signal patterns isdynamically maintained by the communication network 500.

In some embodiments, the reserved subset of signal pattern is designedaccounting for the set of active antenna ports to be used by anotherlayer in low-interference or blank sub frames. In some embodiments, theblank sub frames are almost blank sub frames.

Step 1602

The network node 503 decides an active set of antenna ports from theplurality of antenna ports based on a reserved subset of signal patternor indications to signal patterns associated with at least one layer ofnetwork node. The subset of signal patterns is associated with lowinterference subframes and reserved from the plurality of signalpatterns or indications to signal patterns.

In some embodiments, the deciding and active set of antenna ports isfurther based on low interference subframes. Low interference subframesis associated with time periods with reduced interference.

Step 1603

The network node 503 transmits reference signals from the decided activeset of antenna ports to a user equipment 505 according to the reservedsubset of signal pattern, enabling interference coordination in thecommunication network 500. The transmission of the signal referencesignal is non-dedicated, i.e. it may be received by a plurality of userequipments 505.

To perform the method steps shown in FIG. 16 for enabling interferencecoordination in a communication network 500, the network node 503comprises a network node arrangement as shown in FIG. 17. The networknode 503 comprises a plurality of antenna ports. Each antenna port isconfigured to transmit a reference signal according to a signal pattern.The network node 503 comprises a processor 1701 configured to decide anactive set of antenna ports from the plurality of antenna ports based ona reserved subset of signal patterns associated with at least one layerof network node 503. In some embodiments, the plurality of signalpatterns comprises one or more pattern identities. The subset of signalpatterns associated with low interference subframes and reserved from aplurality of signal patterns or indications to signal patterns. Thenetwork node 503 further comprises a transmitter 1800 configured totransmit reference signals from the decided active set of antenna portsto a user equipment 505 according to the reserved subset of signalpattern, enabling interference coordination in the communication network500. The transmission of the signal reference signal is non-dedicated,i.e. it may be received by a plurality of user equipments 505. In someembodiments, the deciding an active set of antenna ports is furtherbased on blank sub frames. In some embodiments, the blank sub frames arealmost blank sub frames.

In some embodiments, the processor 1701 is further configured to reservea subset of signal patterns for at least one layer of network node 503.The subset of signal patterns is reserved from the plurality of signalpatterns, and the subset of signal patterns is associated with lowinterference subframes.

In some embodiments, the subset of signal patterns is reserved for agroup of network nodes 503. In some embodiments, the group network nodes503 belong to a layer.

In some embodiments, the plurality of signal patterns or indications tosignal patterns is pre-configured in the network node, configured basedon the information received from a second network node within thecommunication network or configured based on the information obtainedfrom a macro cell 501 b associated with a base station 503.

In some embodiments, the reserved subset of signal patterns isdynamically maintained by the communication network 500.

In some embodiments, the reserved subset of signal pattern is designedaccounting for the set of active antenna ports to be used by anotherlayer in low-interference or almost blank sub frames.

As described above, methods and apparatus in accordance with embodimentsherein implement one or more of the following aspects:

-   -   Facilitate control of the set of active antenna ports used for        RS transmissions in order to reduce the RS interference.        -   Including the signaling and the interfaces that may be            involved in the method.    -   Assisted UE measurement processing.        -   Including the signaling and the interfaces that may be            involved in the method.    -   Enhanced cell planning adopted for heterogeneous network        deployments.

Such methods and apparatus have at least the following technicaladvantages:

-   -   Reduced CRS interference in the control region, on CRS, and data        channels leading to the improved system performance and in        particular in heterogeneous deployments.    -   Facilitating UE measurements with some of the disclosed methods        by introducing the new signaling reducing the UE complexity.    -   Reduced over-estimation of the radio channel quality for legacy        macro UEs, which may include low-interference subframes in the        interference measurements, although they will only be scheduled        in subframes with potentially much higher interference.    -   Enhanced cell planning aiming at the improved performance with        heterogeneous deployments.

FIG. 18 is a block diagram of an example of a portion of transmitter1800 for a communication system that uses the signals described above,i.e. the reference signals. The transmitter 1900 may be comprised ine.g. a base station 503, a network node 503 etc. As known for a skilledperson, a communication system is equivalent to a communication network500. Several parts of such a transmitter 1800 are known and describedfor example in Clauses 6.3 and 6.4 of 3GPP TS 36.211. Reference signalshaving symbols as described above are produced by a suitable generator1802 and provided to a modulation mapper 1804 that producescomplex-valued modulation symbols. A layer mapper 1806 maps themodulation symbols onto one or more transmission layers, which generallycorrespond to antenna ports as described above. An Resource Element (RE)mapper 1808 maps the modulation symbols for each antenna port ontorespective Res 1808, and an OFDM signal generator 1810 produces one ormore complex-valued time-domain OFDM signals for eventual transmission.

It will be appreciated that the functional blocks depicted in FIG. 18can be combined and re-arranged in a variety of equivalent ways, andthat many of the functions can be performed by one or more suitablyprogrammed digital signal processors, such as the processor 901illustrated in FIG. 9, processor 1501 illustrated in FIG. 15 and theprocessor 1701 illustrated in FIG. 17. Moreover, connections among andinformation provided or exchanged by the functional blocks depicted inFIG. 18 can be altered in various ways to enable a device to implementthe methods described above and other methods involved in the operationof the device in a digital communication system.

FIG. 19 is a block diagram of an arrangement 1900 in a user equipment505 that may implement the methods described above. It will beappreciated that the functional blocks depicted in FIG. 19 can becombined and re-arranged in a variety of equivalent ways, and that manyof the functions can be performed by one or more suitably programmeddigital signal processors, such as processor 1916 illustrated in FIG. 11and processor 1916 illustrated in FIG. 19. Moreover, connections amongand information provided or exchanged by the functional blocks depictedin FIG. 19 can be altered in various ways to enable a user equipment 505to implement other methods involved in the operation of the userequipment 505.

As depicted in FIG. 19, a user equipment 505 receives a downlink (DL)radio signal through an antenna 1902 and typically down-converts thereceived radio signal to an analog baseband signal in a front endreceiver (Fe RX) 1904. The baseband signal is spectrally shaped by ananalog filter 1906 that has a bandwidth BW₀, and the shaped basebandsignal generated by the filter 1906 is converted from analog to digitalform by an analog-to-digital converter (ADC) 1908.

The digitized baseband signal is further spectrally shaped by a digitalfilter 1910 that has a bandwidth BW_(sync), which corresponds to thebandwidth of synchronization signals or symbols included in the DLsignal. The shaped signal generated by the filter 1910 is provided to acell search unit 1912 that carries out one or more methods of searchingfor cells as specified for the particular communication system, e.g., 3GLTE. Typically, such methods involve detecting predetermined primaryand/or secondary synchronization channel (P/S-SCH) signals in thereceived signal.

The digitized baseband signal is also provided by the ADC 1908 to adigital filter 1914 that has the bandwidth BW₀, and the filtered digitalbaseband signal is provided to a processor 1916 that implements a fastFourier transform (FFT) or other suitable algorithm that generates afrequency-domain (spectral) representation of the baseband signal. Achannel estimation unit 1918 receives signals from the processor 1916and generates a channel estimate H_(i, j) for each of severalsubcarriers i and cells j based on control and timing signals providedby a control unit 1920, which also provides such control and timinginformation to the processor 1916.

The estimator 1918 provides the channel estimates H_(i) to a decoder1922 and a signal power estimation unit 1924. The decoder 1922, whichalso receives signals from the processor 1916, is suitably configured toextract information from RRC or other messages as described above andtypically generates signals subject to further processing in the UE 505(not shown). The estimator 1924 generates received signal powermeasurements, e.g., estimates of reference signal received power (RSRP),received subcarrier power S_(i), signal to interference ratio (SIR),etc. The estimator 1924 can generate estimates of RSRP, reference signalreceived quality (RSRQ), received signal strength indicator (RSSI),received subcarrier power S_(i), SIR, and other relevant measurements,in various ways in response to control signals provided by the controlunit 1920. Power estimates generated by the estimator 1924 are typicallyused in further signal processing in the UE 505.

The estimator 1924 (or the searcher 1912, for that matter) is configuredto include a suitable signal correlator for handling the RS and othersignals described above.

In the arrangement depicted in FIG. 19, the control unit 1920 keepstrack of substantially everything needed to configure the searcher 1912,processor 1916, estimation unit 1918, and estimator 1924. For theestimation unit 1918, this includes both method and cell identity, forreference signal extraction and cell-specific scrambling of referencesignals. Communication between the searcher 1912 and the control unit1920 includes cell identity and, for example, cyclic prefixconfiguration.

The control unit 1920 determines which estimation method is used by theestimator 1918 and/or by the estimator 1924 for measurements on thedetected cell(s) as described above. In particular, the control unit1920, which typically can include a correlator or implement a correlatorfunction, can receive information signaled by the eNB 503 and cancontrol the on/off times of the Fe RX 2004 as described above.

The control unit and other blocks of the UE 505 can be implemented byone or more suitably programmed electronic processors, collections oflogic gates, etc. that processes information stored in one or morememories. The stored information can include program instructions anddata that enable the control unit to implement the methods describedabove. It will be appreciated that the control unit typically includestimers, etc. that facilitate its operations.

The methods and apparatus described can be implemented in heterogeneousdeployments, but they are not limited to them, and neither are theylimited to the 3GPP definition of heterogeneous network deployments. Forexample, the methods and apparatus can be adopted also for traditionalmacro deployments and/or networks operating more than one radio accesstechnology (RAT). The methods are particularly useful for signalstransmitted with a pre-defined time-frequency pattern and a limited setof available patterns, implying high collision probability and thus highinterference in certain parts of the spectrum. LTE cell-specificreference signals (CRS) are an example of such signals.

It will be appreciated that the methods and devices described above canbe combined and re-arranged in a variety of equivalent ways, and thatthe methods can be performed by one or more suitably programmed orconfigured digital signal processors and other known electroniccircuits, e.g., discrete logic gates interconnected to perform aspecialized function, or application-specific integrated circuits. Manyaspects of embodiments herein are described in terms of sequences ofactions that can be performed by, for example, elements of aprogrammable computer system. User equipments 505 embodying embodimentsherein include, for example, mobile telephones, pagers, headsets, laptopcomputers and other mobile terminals, and the like. Moreover, theembodiments herein may additionally be considered to be embodiedentirely within any form of computer-readable storage medium havingstored therein an appropriate set of instructions for use by or inconnection with an instruction-execution system, apparatus, or device,such as a computer-based system, processor-containing system, or othersystem that can fetch instructions from a medium and execute theinstructions.

It will be appreciated that procedures described above are carried outrepetitively as necessary, for example, to respond to the time-varyingnature of communication channels between transmitters and receivers. Inaddition, it will be understood that the methods and apparatus describedhere can be implemented in various system nodes.

To facilitate understanding, many aspects of embodiments herein aredescribed in terms of sequences of actions that can be performed by, forexample, elements of a programmable computer system. It will berecognized that various actions could be performed by specializedcircuits, e.g., discrete logic gates interconnected to perform aspecialized function or application-specific integrated circuits, byprogram instructions executed by one or more processors, or by acombination of both. Wireless devices implementing embodiments hereinmay be included in, for example, mobile telephones, pagers, headsets,laptop computers and other mobile terminals, base stations, and thelike.

Moreover, the embodiments herein can additionally be considered to beembodied entirely within any form of computer-readable storage mediumhaving stored therein an appropriate set of instructions for use by orin connection with an instruction-execution system, apparatus, ordevice, such as a computer-based system, processor-containing system, orother system that can fetch instructions from a storage medium andexecute the instructions. As used here, a “computer-readable medium” canbe any means that can contain, store, or transport the program for useby or in connection with the instruction-execution system, apparatus, ordevice. The computer-readable medium can be, for example but not limitedto, an electronic, magnetic, optical, electromagnetic, infrared, orsemiconductor system, apparatus, or device. More specific examples, anon-exhaustive list, of the computer-readable medium include anelectrical connection having one or more wires, a portable computerdiskette, a random-access memory (RAM), a read-only memory (ROM), anerasable programmable read-only memory (EPROM or Flash memory), and anoptical fiber.

Thus, the embodiments herein maybe embodied in many different forms, notall of which are described above, and all such forms are contemplated tobe within the scope of embodiments herein. For each of the variousaspects of the embodiments, any such form may be referred to as “logicconfigured to” perform a described action, or alternatively as “logicthat” performs a described action.

It should be noted that the word “comprising” does not exclude thepresence of other elements or steps than those listed and the words “a”or “an” preceding an element do not exclude the presence of a pluralityof such elements.

What is claimed is:
 1. A method in a base station for enablinginterference coordination in a communication network, the base stationhaving a plurality of antenna ports that are each configured to transmita reference signal and are each associated with a respective cell, themethod comprising: determining a set of cells where transmission ofreference signals is to be performed from a reduced set of the pluralityof antenna ports, determining a subset of antenna ports in at least onecell of the set of cells to enable interference coordination in thecommunication network; and transmitting the reference signal from thesubset of antenna ports.
 2. The method of claim 1, further comprising:determining a time when the reference signal is to be transmitted from areduced set of antenna ports, wherein the time is associated with lowinterference subframes, and the reference signal is transmitted from thesubset of the antenna ports at the determined time.
 3. The method ofclaim 2, further comprising: informing a user equipment about thedetermined time.
 4. The method of claim 3, further comprising: informinga user equipment about the subset of antenna ports.
 5. The method ofclaim 2, wherein at least one of the determined time and information ofthe subset of antenna ports is obtained from a network node in thecommunication network.
 6. The method of claim 1, further comprising:determining a time interval during which the subset of antenna portsshall apply.
 7. The method of claim 6, further comprising: informing auser equipment about the time interval.
 8. The method of claim 1,further comprising: re-initiating transmission of reference signals fromthe plurality of antenna ports.
 9. The method of claim 8, furthercomprising: informing a user equipment about re-initiated referencesignal transmissions.
 10. The method of claim 1, further comprising:receiving measurements from a user equipment.
 11. The method of claim 1,wherein the subset of antenna ports is configured to avoid interferencefrom an interfering cell or reduce interference to another cell.
 12. Themethod of claim 11, wherein the interference coordination is implementedwith respect to a high interference area of a cell.
 13. The method ofclaim 1, wherein the subset of antenna ports is pre-configured.
 14. Themethod of claim 1, wherein transmissions from the reduced set of antennaports apply to a part of a system bandwidth.
 15. The method of claim 1,wherein transmissions from the reduced set of antenna ports in a cellare periodic or invoked by an event.
 16. A base station for enablinginterference coordination in a communication network, the base stationhaving a plurality of antenna ports that are each configured to transmita reference signal and are each associated with a respective cell, thebase station comprising: a processor configured to determine a set ofcells where transmission of reference signals is to be performed from areduced set of the plurality of antenna ports, and determine a subset ofantenna ports in at least one cell of the set of cells to enableinterference coordination in the communication network; and atransmitter configured to transmit the reference signals from the subsetof antenna ports.
 17. A method in a network node for enablinginterference coordination in a communication network, the network nodebeing associated with a cell, the method comprising: acquiringinformation about a set of interfering cells among a plurality of cells;and transmitting the information about the set of interfering cells to auser equipment to enable interference coordination in the communicationnetwork.
 18. The method of claim 17, wherein the information about theset of interfering cells comprises assistance data.
 19. The method ofclaim 18, wherein the assistance data comprises at least one of a set ofphysical layer cell identities, a transmit bandwidth of an interferingcell, channel-related information, and a number of antenna ports. 20.The method of claim 17, wherein transmitting the information about theset of interfering cells is triggered upon detecting predeterminedinterference conditions for user equipment in the communication network.21. The method according to claim 17, wherein the network node isassociated with an interfering macro cell.
 22. A network node forenabling interference coordination in a communication network, thenetwork node being associated with a cell, the network node comprising:a processor configured to acquire information about a set of interferingcells among a plurality of cells; and one or more antennas configured totransmit information about the set of interfering cells to a userequipment to enable interference coordination in the communicationnetwork, wherein the information transmitted is based on the informationacquired.
 23. The network node of claim 22, wherein the one or moreantennas are further configured to transmit the information about theset of interfering cells when predetermined interference conditions forthe user equipment are detected in the communication network.
 24. Amethod in a network node for enabling interference coordination in acommunication network, wherein the network node includes a plurality ofantenna ports that are each configured to transmit a reference signalaccording to a signal pattern, the method comprising: deciding an activeset of antenna ports from the plurality of antenna ports based on areserved subset of signal patterns associated with at least one layer ofnetwork nodes, the reserved subset of signal patterns being associatedwith low interference and reserved from a plurality of signal patternsor indications to signal patterns; and transmitting reference signalsfrom the active set of antenna ports according to the reserved subset ofsignal patterns, thereby enabling interference coordination in thecommunication network.
 25. The method of claim 24, further comprising:reserving a subset of signal patterns for at least one layer of networknodes, wherein the subset of signal patterns is reserved from theplurality of signal patterns or indications to signal patterns, and thesubset of signal patterns is associated with low interference.
 26. Themethod of claim 25, wherein the subset of signal patterns is reservedfor a group of network nodes.
 27. The method of claim 26, wherein thegroup network nodes belongs to a layer.
 28. The method of claim 24,wherein the plurality of signal patterns or indications to signalpatterns is pre-configured in the network node, obtained from a secondnetwork node, or obtained from a macro cell.
 29. The method of claim 24,wherein the set of active antenna ports is decided based onlow-interference subframes.
 30. The method of claim 24, wherein thereserved subset of signal patterns is dynamically maintained by thecommunication network.
 31. The method of claim 24, wherein the reservedsubset of signal patterns is based on the set of active antenna ports tobe used by another layer in low-interference subframes.
 32. A networknode for enabling interference coordination in a communication network,the network node having a plurality of antenna ports that are eachconfigured to transmit a reference signal according to a signal pattern,the network node comprising: a processor configured to decide an activeset of antenna ports from the plurality of antenna ports based on areserved subset of signal patterns associated with at least one layer ofnetwork node, wherein the subset of signal patterns is associated withlow interference and is reserved from a plurality of signal patterns orindications to signal patterns; and a transmitter configured to transmitreference signals from the active set of antenna ports according to thereserved subset of signal patterns, thereby enabling interferencecoordination in the communication network.
 33. The network node of claim32, wherein the plurality of signal patterns or indications to signalpatterns is pre-configured in the network node or configured based oninformation received from a second network node in the communicationnetwork or configured based on information obtained from a macro cellassociated with a base station.